The Type XXI U-Boat had a rubber coating, making it difficult to detect. Also, its streamlined design allowed it to move faster underwater than it could while surfaced. It had a sophisticated electronics system.
Also, if you’re giving Germany the Type XXI U-boat in 1943, then you should also give it the kinds of torpedoes it had available in 1945.
Hmm maybe the underwater launchable Do-Werfers as well, too?!
German had some successful underwater launchings with, I think 15 or 28cm Do-werfers ,in 1942 near the Penemünde area.